A firearm is made of the following.
Propellant. The powder, black or smokeless. black is more primitive and a low powered explosive, producing velocities up to 2000fps generally. Smokeless powders(modern powder) are a propellant not an explosive and create thrust by great and controlled release of energy with velocities up to 5000fps or so.
Projectile. What gets fired out of the gun. May be a pellet-round object, generally lead or steel or bullet-any other shape, generally several times longer than it is wide.
Ignition. The means for igniting the choice of propellant. For blackpowder muzzleloaders(where everything is poured into the chambre through the muzzle end of the barrel) it goes right back to the ancient method of lighting cannon fuse hanging off the side of the gun through the early lock, wheel, flintlock, matchlock etc to percussion caps(like on a toy gun).
For cartrdige rifles it either includes a priming compound included in the manufacture of the case, which is set off when struck by a firing pin. Or a 'primer' which is a separate object placed into a special hole in the rear of the cartridge, for the same effect. Lastly there are electronically ignited cartridges as well.
Reaction chamber-where the pressure is generated to propel the projectile. Either by a blackpowder explosion or pressure peak within a smokeless cartridge case.
Delivery system. The barrel. Either smoothbore(no rifling, poor accuracy, or for shotgun pellets) or rifled- using grooves in the barrels internal metal to spin the bullet, thus creating gyroscopic stabilisation and greater accuracy(like spinning a football when you throw it).
sighting system-a means for putting the projectile on target.
furniture- added bits and pieces which make up the rest of the gun, so you can hold and fire it. Such as stocks, fore ends, slings etc.